Three-layer temperature structure of oceans
Topic: Oceans: Hydrological Cycle, Relief of the Ocean Floor, Temperature and Salinity · NCERT: Class 11, Ch 12 "Water (Oceans)"
Meaning
In low and middle latitudes, ocean water forms three temperature layers from top to bottom:
- Warm surface layer: about 500 m thick, at 20–25°C. It lasts all year in the tropics but forms only in summer in mid-latitudes.
- Thermocline layer: about 500–1,000 m thick, where temperature falls rapidly with depth.
- Cold deep layer: very cold water that reaches down to the ocean floor.
The layers form because the sun heats the sea only from above, and the heat mixes downwards slowly. Polar seas have only one cold layer from the surface to the floor. The real depth of each layer changes with latitude and season.
Example
In the tropical Indian Ocean around India, the warm surface layer lasts all year. In the Southern Ocean near Antarctica, the water stays cold from the surface to the bottom.
Don't confuse with
- Thermocline: the thermocline is only the middle layer, where temperature drops sharply. The three-layer structure describes the whole water column.